Dunno - I always dig 'em up and soak the roots in a bucket of water, and then separate them, much easier to do then. If yours is in flower, or about to flower, now is not the time to separate it - wait till its finished flowering.

Agree with Bamboo give them a soak and you should be able to tease the plants apart. Best done late summer.

I've always used a pair of secateurs to cut into the crown and carefully cut between the leave stems. That's always worked for me. Not heard of the water soaking method before.